It was the third recent shooting of a child in western Washington.
"It is incredible in light of the other ones," said Tacoma police Officer Naveed Benjamin. "You would think people would take more care, not less."
The family was traveling and had stopped for gas about 12:30 a.m. at the Tacoma station off Interstate 5. The man put his pistol under the seat and got out to pump gas while the boy's mother went inside the station, Mr Benjamin said.
They left her son and her boyfriend's 4- or 5-year-old daughter in the car, Mr Benjamin said.
The boy climbed out of his child seat, found the gun and shot himself. He was declared dead at a hospital.
The girl was not injured.
Detectives questioned the parents and have called the shooting a tragic accident, Mr Benjamin said.
The family has not been named. They are not from Tacoma and their car has Oregon plates.
The shooting follows the death of the 7-year-old daughter of a Marysville police officer in Stanwood on Saturday when a sibling found a gun and fired while the parents were out of their car. And on February 22, an 8-year-old girl was critically wounded in a Bremerton classroom when a gun fired from the backpack of a 9-year-old boy as he put it on a desk.
"It's another tragedy in a very short period of time," Mr Benjamin said.
It highlights the need for people to secure guns, he said.
"You can't predict what children are going to do," he said. "You need to unload and lock it up if you're not carrying it."
"And keep it out of the hands of children," Mr Benjamin said. "It's really not that hard to practice firearm safety."
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